"D"
Scales 

D 
The D fundamental scale is normally
positioned as the top scale on bottom rule. It runs from 1
to 10. Gauge marks are located at C (√(4/π)), ⑫ (dozen), Ⓖ (gross), ρ (π/180), ρ'' (180*60*60)/π), ρ' (180*60)/π), used in radians to degrees conversions, π, τ, ρ⁰ (180/π), and C^{1} (√(40/π)). Formula: D = X 
D_{1}  The D_{1} scale is a ranged
fundamental D scale, ranging from 1 to √10. Gauge mark is present at √10. Formula: D_{1} = √10 × X 
D_{2}  The D_{2} scale is a ranged
reciprocal fundamental D scale, ranging from √10 to 1. Gauge mark is present at √10. Formula: D_{2} = 1 / ( √10 × X ) 
DF  The DF scale, like the CF scale, is a folded
D scale, with the fold at π. It is found on the top or
bottom rule, and is used for calculations involving π. Gauge marks are located at π and τ. Formula: DF = π × X 
DF/m  The DF/m scale is a folded D scale like the
DF scale, with the fold is at 2.3 (natural log of 10).
This scale is useful when converting results between base 10
and base e.
This scale is usually located on the bottom rule, where the
CF/m scale would be located on the slide. Gauge marks are located at π and τ. Formula: DF/m = ( log_{e} 10 ) × X 
DI 
The DI scale is an inverse D scale, reversed
in direction from the D scale (reciprocal scale). Gauge marks are located at π and τ. Formula: DI = 1 / X 
DIF  The DIF scale is another reciprocal folded
fundamental scale, like the CIF scale. This scale is
normally on the top or bottom rule, and is folded at π, just
like the CIF scale. Gauge marks are located at π and τ. Formula: DIF = 1 / ( π × X ) 